The present invention is related to a method of modifying the behavior of gas hydrates to inhibit the plugging of conduits by using a novel amine which includes at least one ether linkage.
Gas hydrates are clathrates (inclusion compounds) of gases in a lattice consisting of water molecules. Low-boiling hydrocarbons, such as methane, ethane, propane, butane and iso-butane, are present in natural gas and also in crude oil. Because water may also be present in varying amounts in natural gas and crude oil, the low-boiling gas mixture, under conditions of elevated pressure and reduced temperature, tends to form gas hydrate crystals. The maximum temperature at which gas hydrates can form strongly depends on the pressure of the system. For example, ethane at a pressure of approximately 1 MPa can form hydrates only at temperatures below 4xc2x0 C., whereas at a pressure of 3 MPa stable hydrates can be present at temperatures as high as 14xc2x0 C. With respect to this strong dependence of the hydrate melting point on pressure, hydrates differ markedly from ice.
As described by M. von Stackelberg and H. R. Muller (Z. Electrochem. 1954 5825), methane and ethane hydrates form cubic lattices having a lattice constant of 1.2 nm (See, for example, hydrate structure I). The lattice constant of the cubic propane and butane gas hydrates is 1.73 nm (See, for example, hydrate structure II). However, the presence of even small amounts of propane in a mixture of low-boiling hydrocarbons will result in the formation. of gas hydrates having hydrate structure II (J. H. van der Waals and J. C. Platteeuw, Adv. Chem. Phys. 2 1959 1).
It has been known for a long time that gas hydrate crystals, when allowed to form and grow inside a conduit such as a pipeline, tend to block or even damage the conduit. To prevent such blocking, the following thermodynamic measures are possible in principle: removing free water, maintaining elevated temperatures and/or reduced pressures, or adding melting point depressants (antifreeze). In practice, the last-mentioned measure, i.e., the addition of antifreezes, is most often applied. However, the antifreezes, such as the lower alcohols and glycols, have to be added in substantial amounts (several tens of a percent bet weight of the water present) to be effective. An additional disadvantage of such high-amounts is that recovery of the antifreezes is usually required during subsequent processing of the mixtures. Furthermore, at practical dosages, antifreezes may only reduce the kinetics of forming hydrates and, if an extended interruption in flow occurs, damaging hydrates may still form.
An attractive alternative to the anti-hydrate measures described above, particularly the antifreezes, is to use anti-agglomeration inhibitors which modify the hydrate crystal. This crystal modification does not prevent formation of gas hydrates. Instead, anti-agglomeration inhibitors allow small hydrate particles to form which can flow with the oil or gas.
Plants, and animals such as insects and cold-water fish, are known to protect themselves from freezing, both by antifreezes such as glycols and by special peptides and glycopeptides (termed Antifreeze Proteins, AFP""s and Antifreeze Glycoproteins, AFGP""s) which interfere with ice crystal growth (A. L. de Vries, Comp. Diochem. Physiol, 73 1982 627). Such cold-water fish peptides and glycopeptides have been said to be effective in interfering with the growth of gas-hydrate crystals. However, their production and use for this purpose are considered to be uneconomical.
In PCT Patent Application EP93/01519, the use of polymers and copolymers of N-vinyl-2-pyrrolidone for inhibiting the formation, growth and/or agglomeration of gas hydrate crystals is disclosed.
U.S. Pat. No. 5,460,728 discloses that certain alkylated ammonium, phosphonium and sulphonium compounds are effective in interfering with the growth of gas hydrate crystals and are useful in inhibiting the plugging by gas hydrates of conduits containing low-boiling hydrocarbons and water. The compounds have the formula:
(R3xe2x80x94X(R1)xe2x80x94R2)+Yxe2x88x92
wherein R1, R2 and R3 are independently selected from the group consisting of normal and branched alkyl groups containing at least 4 carbon atoms; X is S, Nxe2x80x94R4, or Pxe2x80x94R4 where R4 or Pxe2x80x94R4 is hydrogen or an organic substituent which may contain one or more hetero atoms such as O and can be in particular alkyl, alkenyl, aryl, arylalkyl, arylalkenyl, alkylaryl, alkenylaryl, or glycol having 1 to 20 carbon atoms; and Yxe2x88x92 is an anion. The preferred ammonium compounds are said to be those compounds in which R4 is alkyl or alkenyl containing 8 to 20 carbon atoms.
U.S. Pat. No. 5,460,728 further discloses that compounds wherein R4 contains more than 12 carbon atoms can provide additional properties, in addition to the inherent hydrate crystal growth-inhibiting properties, such as surface-active properties which help to emulsify the aqueous and hydrocarbon phases (as water-in-oil or oil-in-water emulsions), thereby keeping the concentration of water available for hydrate forming at the conduit wall small. Surface-active properties also help to do the following:
Concentrate the preferred compound near the water-hydrocarbon interfaces, where hydrate formation is most pronounced, thereby raising the local concentration of ions to the freezing-point depressing level.
Modify the structure of water near the hydrocarbon-water interface in such a way that the formation of hydrate crystals is hindered.
Impede further access of water molecules to the hydrate crystal after attachment of the subject compound to the hydrate crystals.
Prevent agglomeration of hydrate crystals by making their surface hydrophobic.
Adhere the subject compound to the conduit wall, thereby preventing the adhesion of hydrates thereto.
The amount of the preferred compounds to be used in the process according to U.S. Pat. No. 5,460,728 is generally between 0.05 and 5 wt. %, preferably between 0.1 and 0.5 wt. %, based on the amount of water in the hydrocarbon-containing mixture.
U.S. Pat. No. 5,879,561 discloses a method for inhibiting the plugging of a conduit containing a flowing mixture of hydrocarbons having from 1 to 8 carbon atoms and water. The method includes the addition of a hydrate formation inhibitor having the formula: 
wherein two of R1-R4 are independently normal or branched alkyls having 4 or 5 carbon atoms; two of R1-R4 are independently organic moieties having at least 8 carbon atoms and may contain ester moieties to improve biodegradability; A represents a nitrogen or phosphorus atom; and Y is an anion.
While the prior art compounds may be satisfactory to some extent, there is still a need for compounds which are more hydrophilic, thus providing better water solubility and better dispersibility in antifreeze solutions and which are liquid at lower temperatures thereby providing a higher degree of performance while requiring reduced amounts of the active compound. Furthermore, there is still a need for improved compounds which inhibit the plugging of conduits at lower temperatures, or higher pressures, for longer periods of time than the existing compounds employed in the prior art. It is therefore an object of the present invention to provide improved compounds to inhibit formation of hydrates in streams containing at least some light hydrocarbons and water. It is a further object of the present invention to provide such a method wherein a high concentration of additive is not required.
An even further embodiment of the present invention is to provide improved compounds which do not interfere with the demulsification of the oil when the pressure of the system has been released.
One aspect of the present invention is directed to amine compounds which include ether linkages. Specifically, the amine compounds of the present invention have the following formula:
(RAxe2x80x94Oxe2x80x94BTxe2x80x94Q)sxe2x80x94N+(Cxe2x80x94Cxe2x80x94CN+REkRD)xHv(RB)y(RC)p(RD)zAwxe2x80x83xe2x80x83(1)
wherein RA is selected from the group consisting of alkyl, alkenyl, aryl, arylalkyl, arylalkenyl, alkylaryl, and alkenylaryl groups and glycols having from 1 to 24, preferably from 3-20, and more preferably from 7 to 18, carbon atoms and optionally contains at least one hetero atom such as oxygen, nitrogen or sulfur;
Q is ethylene, propylene or butylene which may be linear or branched, preferably Q is propylene;
x is 0 to 4;
y and z are each independently 0 to x+1;
w is 1 to x+1;
v is a number which saturates the nitrogen atom;
k and p are independently 0 or 1, with the proviso that k+p is greater than or equal to 1;
s is 1, 2 or 3, preferably 1 or 2;
T is from 0 to 20;
each RE, RB and RC is independently selected from the group consisting of straight and branched chain alkyl groups containing 2 to 6 carbon atoms which may optionally be substituted with phenyl, and RE can be further (RAxe2x80x94Oxe2x80x94BTxe2x80x94Q);
each RD is independently selected. from the group consisting of hydrogen and straight and branched chain alkyl groups containing 1 to 6 carbon atoms which may optionally be substituted with phenyl;
B is a residual of ethylene oxide, propylene oxide or butylene oxide; and
A is an anion.
Another aspect of the present invention is directed to a method for inhibiting the plugging of a conduit, the conduit containing a flowing mixture comprising an amount of hydrocarbons having from one to five carbons and an amount of water wherein the amounts of hydrocarbons and water could form hydrates at conduit temperatures and pressures, the method comprising the steps of:
adding to the mixture an amount of a hydrate formation modifier component of formula (1) in an amount effective to substantially inhibit the formation and/or the agglomeration of hydrates in the mixture at conduit temperatures and pressures; and flowing the mixture containing the hydrate formation modifier through the conduit.
The inventive compounds of formula (1) can also be used to reduce the agglomerization of hydrates and/or to delay the formation of hydrates. In this embodiment of the present invention, an effective amount of at least one compound of formula (1) is added to system which may otherwise form hydrates and their agglomerates.
As stated above, the present invention is directed to ether-containing compounds of formula (1) wherein RA is selected from the group consisting of alkyl, alkenyl, aryl, arylalkyl, arylalkenyl, alkylaryl, and alkenylaryl groups and glycols having from 1 to 24, preferably. from 3-20, and more preferably from 7 to 18, carbon atoms and optionally contains at least one hetero atom such as oxygen, nitrogen or sulfur;
Q is ethylene, propylene or butylene which may be linear or branched, preferably Q is propylene;
x is 0 to 4;
y and z are each independently 0 to x+1;
w is 1 to x+1;
v is a number which saturates the nitrogen atom;
k and p are independently 0 or 1, with the proviso that k+p is greater than or equal to 1;
s is 1, 2 or 3, preferably 1 or 2;
T is from 0 to 20;
each RE, RB and RC is independently selected from the group consisting of straight and branched chain alkyl groups containing 2 to 6 carbon atoms which may optionally be substituted with phenyl, and RE can be further (RAxe2x80x94Oxe2x80x94BTxe2x80x94Q);
each RD is independently selected from the group consisting of hydrogen and straight and branched chain alkyl groups containing 1 to 6 carbon atoms which may optionally be substituted with phenyl;
B is a residual of ethylene oxide, propylene oxide or butylene oxide; and
A is an anion.
More in particular, RA is preferably linear or branched alkyl having 1 to 24, preferably 3 to 20 and more preferably 7 to 18 carbon atoms. RB, RC and RD are preferably independently selected from the group of ethyl, propyl, n-butyl and hydrogen. In some embodiments, each RD can also be methyl. The anions of the compounds according to the invention can be broadly chosen. Examples include anions such as hydroxide, carboxylate (such as acetate, propionate, butyrate, or benzoate), halide, sulfate, and organic sulfate. Preferred anions are bromide and alkylsulfates. Of the preferred anions, it is most preferably that the anion is bromide, methylsulfate or ethylsulfate.
The most preferred compounds of U.S. Pat. No. 5,460,728 contain either an oleyl or a C12 to C18 linear fatty moiety which renders the compounds quite hydrophobic. In the present invention, it has surprisingly been discovered that by replacing the fatty moiety with an oxypropylalkyl moiety of a similar range of molecular weight the solubility and the water dispersibility are improved. Surprisingly the resulting compound is not only more dispersible but is in fact, in some instances, soluble in water at room temperature. Even more surprisingly, not only does this improve the ease of application but the effectiveness as a gas hydrate inhibitor is improved significantly.
The compounds of formula (1) according to the present invention can be prepared by procedures which are known in the art using starting materials i.e., reactants which are simple and abundantly available. For instance, the corresponding ether amine can be prepared or acquired and quaternized with a halide such as RD=Br in a solvent such as isopropanol in the presence of a base such as sodium bicarbonate.
When the compounds of formula (1) are used for inhibiting the plugging of a conduit, at least one compound of formula (1) can be added to the mixture of hydrocarbons and water as a liquid or, preferably, in a concentrated solution. The amount of compounds of formula (1) present should comprise 0.05 to 5.0 wt. % based on the weight of water present. Preferably, the amount thereof should be 0.1 to 1.0 wt. % based on the weight of water present. When used in this fashion, these compounds will reduce the speed and/or extent to which gas hydrates agglomerates are formed.
When the compounds of formula (1) are used in reducing the agglomerization of hydrates and/or the delay of hydrate formation, at least one compound of formula (1) is added to the hydrate(s) in an amount such that the compound of formula (1) is present in a concentration of from about 0.05 to about 10, preferably 0.5 to 3, weight % on a solvent-free basis.